The present invention relates to heating systems for furnaces which heat work material and especially, but not limited to, high pressure furnaces commonly refered to as hot isostatic pressure (HIP) furnaces.
Among the types of furnaces which heat work material is the so-called hot isostatic pressure furnace which will be referred to hereafter as a high pressure furnace to distinguish over furnaces which do not require a heavy duty outer pressure vessel, although the present invention is not limited to use with the latter type, as will be made apparent hereinafter.
Conventional hot isostatic pressure furnaces provide an enclosed work chamber and maintain within the chamber pressures and temperatures sufficient to effect solid state bonding of materials.
Such high pressure furnaces generally comprise a heavy duty outer pressure wall and a removable furnace assembly located therein. The furnace assembly includes a thermal barrier surrounding an inner shell, the latter defining the work chamber and usually carrying a heat-generating system. A pressure medium, for example inert gases like helium or argon, is conducted through the outer pressure wall and eventually to the work chamber to raise the pressure to high levels therein. Simultaneously, the temperature is raised by the heating system. The high pressure forces mating surfaces of the work materials into intimate contact for a sufficient time to cause solid state bonding.
A high pressure furnace can be used to bond metallic, cermet, and ceramic powders or preforms, among other materials, into fully dense structures.
High pressure furnaces are exemplified by that disclosed in U.S. Pat. No. 3,427,011 issued to Boyer et al on Feb. 11, 1969. While the high pressure furnace described in that patent has proven to be highly useful, substantial room for improvement remains.
One of the significant factors encountered in the design of a furnace of the type which heats work materials relates to the size of the volumes to be heated and/or pressurized. In the design of such a furnace, a work space of a certain minimal volume is usually required. Additional space, i.e., non-critical space, is unnecessary from a heating (and/or pressurizing) performance standpoint but may be necessary as a practical matter for containing various components of the actuating mechanism, such as the heating system. However, the presence of such additional space maximizes overall energy demands in order to achieve the desired temperature and/or pressure within the work space, as well as maximizing the overall size of the structure, including the outer confining wall. For this reason, it would be highly desirable to minimize the excess space surrounding the work space.
As noted above, among the components which create excess space is the heating system. In this regard, it has been heretofore proposed, in the above mentioned Boyer et al patent for example, to wind an electrical heating wire in vertical serpentine manner around vertically spaced insulators which are connected in cantilever fashion to the outer side of the shell that defines the work chamber. Such an arrangement necessitates the use of a relatively sturdy (and thus relatively thick-walled) shell. Moreover, the insulators require fasteners, washers etc. which further enlarge the diametrical dimension. Moreover, thick-walled shells are costly to fabricate because of difficulties experienced in rolling metal of such thicknesses.
Another area where improvements can be made to furnaces of the type which heat work materials, involves the time and effort required for assembly and dissassembly of the heating system. In systems of the type described in the above-captioned Boyer et al patent, a plurality of insulator members are individually secured to the outer wall of the chamber defining shell, around which elements a heating wire is to be wound. The need for fastening each insulating member in place involves considerable time and labor. Moreover, it will be appreciated that considerable difficulty is involved in thereafter obtaining access to the heating element and insulator members for maintenance or replacement purposes.
An additional area of concern in the design of such furnaces relates to the normal tendency for thermal expansion and/or contraction of the heating element to occur. One previous manner of dealing with this problem involves the provision of a relatively loose mounting of the heating wire on the insulator members. Such an arrangement is intended to accommodate expansion and contraction of the wire. However, in practice, it can be rather difficult to achieve an optimum amount of "looseness" of the wire to assure that the wire is loose enough to prevent excessive tensioning of the wire on the insulator members when the wire is contracted, and yet is not so loose as to lose contact with the insulator members and short circuit against the chamber-defining shell.
It is, therefore, an object of the present invention to minimize or obviate problems of the type discussed above.
It is another object of the invention to provide a novel heating system for a furnace of the type which heats work material, especially, but not limited to, high pressure furnaces.
It is a further object of the invention to minimize non-critical space within such furnaces.
It is another object of the invention to simplify the installation and removal of a heating system for such furnaces.
It is additional object of the invention to accommodate unconstrained thermal expansion and contraction of a furnace electrical heating element without the danger of short circuiting.
It is another object of the invention to minimize the power requirements of such a furnace as well as to simplify the maintenance of the heating system thereof.